Modular Arithmetic

16 questions

Question 1Question →

Find the smallest non-negative integer xx that satisfies the linear modular congruence 3x≡8(mod11)3x \equiv 8 \pmod{11}.

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Answer: 10

Answer

The smallest non-negative integer xx is 10.
Evaluating 3x≡8(mod11)3x \equiv 8 \pmod{11} by testing multiples of 1111 added to 88 gives 3030, which divided by 33 yields x=10x = 10. Since 10∈[0,10]10 \in [0, 10], it is the canonical solution.

Step-by-Step Solution

1
Convert the modular congruence into an algebraic equation
3x=8+11k3x = 8 + 11k for an integer kk
By definition of congruence modulo 1111, 3x−83x - 8 must be a multiple of 1111.
2
Find the smallest integer k≥0k \ge 0 such that 8+11k8 + 11k is divisible by 3
When k=2k = 2, 8+11(2)=308 + 11(2) = 30
3030 is divisible by 33 (30/3=1030 / 3 = 10).
3
Divide by 3 to isolate xx
x=10x = 10
3(10)=30≡8(mod11)3(10) = 30 \equiv 8 \pmod{11}.

Key Concept

Linear Modular Congruence
Estimated Time:1m 15s
Question 2Question →

What is the value of (−38)(mod7)(-38) \pmod{7} expressed in standard non-negative remainder form?

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Answer: 4

Answer

4
The value 44 is correct because −38=7×(−6)+4-38 = 7 \times (-6) + 4. The remainder 44 lies in the standard non-negative range 0≤r<70 \le r < 7. Alternatively, adding multiples of 77 to −38-38 gives −38+35=−3-38 + 35 = -3, and adding 77 once more gives −3+7=4-3 + 7 = 4.

Step-by-Step Solution

1
Find the largest multiple of the modulus 77 that is less than or equal to −38-38.
The multiple is 7×(−6)=−427 \times (-6) = -42.
Modular arithmetic requires the remainder rr to satisfy 0≤r<70 \le r < 7 in standard form.
2
Calculate the remainder by subtracting the multiple from the dividend.
−38−(−42)=−38+42=4-38 - (-42) = -38 + 42 = 4.
The remainder is the non-negative difference between the number and the multiple of the modulus.

Key Concept

Modular Arithmetic with Negative Numbers
Estimated Time:1m 0s
Question 3Question →

If 5x≡2(mod11)5x \equiv 2 \pmod{11}, what is the value of (x3−4x)(mod11)(x^3 - 4x) \pmod{11} expressed in standard non-negative remainder form?

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Answer: 7

Answer

7
Solving the linear congruence 5x≡2(mod11)5x \equiv 2 \pmod{11} gives x≡7(mod11)x \equiv 7 \pmod{11}. Substituting x=7x = 7 into the expression (x3−4x)(x^3 - 4x) yields 73−4(7)=343−28=3157^3 - 4(7) = 343 - 28 = 315. Dividing 315 by 11 gives a quotient of 28 and a remainder of 7. Alternatively, working entirely modulo 11 gives 2−6=−4≡7(mod11)2 - 6 = -4 \equiv 7 \pmod{11}. Thus, the canonical non-negative remainder is 7.

Step-by-Step Solution

1
Find the modular inverse of 5 modulo 11
The inverse is 9, since 5×9=45≡1(mod11)5 \times 9 = 45 \equiv 1 \pmod{11}.
To isolate xx in the linear congruence 5x≡2(mod11)5x \equiv 2 \pmod{11}, multiply both sides by the modular inverse of 5.
2
Solve for x modulo 11
x≡2×9=18≡7(mod11)x \equiv 2 \times 9 = 18 \equiv 7 \pmod{11}.
Simplifying 18 modulo 11 gives the canonical value of xx.
3
Evaluate x3−4xx^3 - 4x modulo 11
x3≡73=343≡2(mod11)x^3 \equiv 7^3 = 343 \equiv 2 \pmod{11} and 4x≡4(7)=28≡6(mod11)4x \equiv 4(7) = 28 \equiv 6 \pmod{11}, so x3−4x≡2−6=−4(mod11)x^3 - 4x \equiv 2 - 6 = -4 \pmod{11}.
Substitute x≡7x \equiv 7 into the polynomial expression and reduce each term modulo 11.
4
Convert negative remainder to canonical non-negative form
−4+11=7(mod11)-4 + 11 = 7 \pmod{11}.
Modular remainders must be expressed within the standard range [0,10][0, 10].

Key Concept

Linear modular congruences and negative remainder canonical reduction
Estimated Time:2m 0s
Question 4Question →

What is the smallest non-negative integer kk that satisfies the modular congruence 799+k≡−15(mod11)7^{99} + k \equiv -15 \pmod{11}?

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Answer: 10

Answer

The correct answer is 10.
Using Fermat's Little Theorem, 710≡1(mod11)7^{10} \equiv 1 \pmod{11}, which simplifies 799(mod11)7^{99} \pmod{11} to 79≡8(mod11)7^9 \equiv 8 \pmod{11}. Reducing the right-hand side gives −15≡7(mod11)-15 \equiv 7 \pmod{11}. The modular equation 8+k≡7(mod11)8 + k \equiv 7 \pmod{11} yields k≡−1(mod11)k \equiv -1 \pmod{11}. Adding the modulus 1111 gives the canonical positive remainder 1010.

Step-by-Step Solution

1
Simplify 799(mod11)7^{99} \pmod{11} using Fermat's Little Theorem.
710≡1(mod11)7^{10} \equiv 1 \pmod{11}, so 799=(710)9×79≡19×79≡79(mod11)7^{99} = (7^{10})^9 \times 7^9 \equiv 1^9 \times 7^9 \equiv 7^9 \pmod{11}.
Since 1111 is prime and gcd⁡(7,11)=1\gcd(7, 11) = 1, Fermat's Little Theorem allows exponent reduction modulo 1010.
2
Compute 79(mod11)7^9 \pmod{11}.
71≡77^1 \equiv 7, 72=49≡57^2 = 49 \equiv 5, 74≡52=25≡37^4 \equiv 5^2 = 25 \equiv 3, 75≡7×3=21≡107^5 \equiv 7 \times 3 = 21 \equiv 10, 79=75×74≡10×3=30≡8(mod11)7^9 = 7^5 \times 7^4 \equiv 10 \times 3 = 30 \equiv 8 \pmod{11}.
Repeated squaring and modular multiplication efficiently reduces 797^9 modulo 1111.
3
Reduce the right-hand side −15(mod11)-15 \pmod{11}.
−15=−2(11)+7≡7(mod11)-15 = -2(11) + 7 \equiv 7 \pmod{11}.
Converting negative numbers into the standard non-negative remainder range [0,10][0, 10].
4
Substitute remainders into the congruence and solve for kk.
8+k≡7  ⟹  k≡7−8=−1(mod11)8 + k \equiv 7 \implies k \equiv 7 - 8 = -1 \pmod{11}.
Linear algebraic rearrangement in modular arithmetic.
5
Convert the negative remainder −1-1 to canonical non-negative form.
k=−1+11=10k = -1 + 11 = 10.
The standard remainder must satisfy 0≤k<110 \leq k < 11.

Key Concept

Modular Exponentiation & Negative Remainder Reduction
Question 5Question →

Find the smallest positive integer nn that simultaneously satisfies the linear modular congruences 3n≡5(mod13)3n \equiv 5 \pmod{13} and 4n≡2(mod9)4n \equiv 2 \pmod{9}.

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Answer: 32

Answer

The smallest positive integer satisfying both congruences is 32.
Solving the first congruence 3n≡5(mod13)3n \equiv 5 \pmod{13} gives n≡6(mod13)n \equiv 6 \pmod{13} (since 3×9≡1(mod13)3 \times 9 \equiv 1 \pmod{13} and 5×9=45≡6(mod13)5 \times 9 = 45 \equiv 6 \pmod{13}), which means nn can be written as 13k+613k + 6. Substituting this into the second congruence 4n≡2(mod9)4n \equiv 2 \pmod{9} yields 4(13k+6)≡2(mod9)  ⟹  52k+24≡2(mod9)4(13k + 6) \equiv 2 \pmod{9} \implies 52k + 24 \equiv 2 \pmod{9}. Reducing the coefficients modulo 9 gives 7k+6≡2(mod9)  ⟹  7k≡−4≡5(mod9)7k + 6 \equiv 2 \pmod{9} \implies 7k \equiv -4 \equiv 5 \pmod{9}. Multiplying by 4 (the modular inverse of 7 modulo 9) yields k≡20≡2(mod9)k \equiv 20 \equiv 2 \pmod{9}. Setting k=2k = 2 yields the smallest positive integer n=13(2)+6=32n = 13(2) + 6 = 32.

Step-by-Step Solution

1
Solve the first modular congruence 3n≡5(mod13)3n \equiv 5 \pmod{13} for nn
n≡6(mod13)n \equiv 6 \pmod{13}, which implies n=13k+6n = 13k + 6
Multiplying both sides by the modular inverse of 3 modulo 13 (which is 9) isolates nn.
2
Solve the second modular congruence 4n≡2(mod9)4n \equiv 2 \pmod{9} for nn
n≡5(mod9)n \equiv 5 \pmod{9}
Multiplying both sides by the modular inverse of 4 modulo 9 (which is 7) isolates nn.
3
Substitute n=13k+6n = 13k + 6 into n≡5(mod9)n \equiv 5 \pmod{9} and simplify modulo 9
4k≡8(mod9)4k \equiv 8 \pmod{9}
Reducing 13 modulo 9 yields 4k4k, and subtracting 6 from 5 yields −1≡8(mod9)-1 \equiv 8 \pmod{9}.
4
Solve for kk and calculate the smallest positive integer nn
k≡2(mod9)k \equiv 2 \pmod{9}, giving n=13(2)+6=32n = 13(2) + 6 = 32
Setting the integer parameter kk to its minimum non-negative value 22 provides the smallest positive integer solution.

Key Concept

System of Linear Modular Congruences and Modular Inverses
Question 6Question →

If xx is the smallest positive integer satisfying the modular congruence 2x≡7(mod11)2^x \equiv 7 \pmod{11}, what is the value of (3x2−4x+5)(mod11)(3x^2 - 4x + 5) \pmod{11} expressed in standard non-negative remainder form?

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Answer: 3

Answer

The smallest positive integer exponent satisfying 2x≡7(mod11)2^x \equiv 7 \pmod{11} is x=7x = 7. Substituting x=7x = 7 into 3x2−4x+53x^2 - 4x + 5 yields 124124, which simplifies to 3(mod11)3 \pmod{11}.
Evaluating powers of 2 modulo 11 shows that 27=128≡7(mod11)2^7 = 128 \equiv 7 \pmod{11}, giving x=7x = 7. Substituting x=7x = 7 into 3x2−4x+53x^2 - 4x + 5 gives 124124, which leaves a remainder of 33 when divided by 1111.

Step-by-Step Solution

1
Find the smallest positive integer exponent xx satisfying 2x≡7(mod11)2^x \equiv 7 \pmod{11}
x=7x = 7
Evaluating consecutive powers of 2 modulo 11 shows 21≡22^1 \equiv 2, 22≡42^2 \equiv 4, 23≡82^3 \equiv 8, 24≡52^4 \equiv 5, 25≡102^5 \equiv 10, 26≡92^6 \equiv 9, and 27≡72^7 \equiv 7, making x=7x = 7 the smallest positive integer power.
2
Substitute x=7x = 7 into the expression 3x2−4x+53x^2 - 4x + 5
124
Direct substitution gives 3(7)2−4(7)+5=3(49)−28+5=147−28+5=1243(7)^2 - 4(7) + 5 = 3(49) - 28 + 5 = 147 - 28 + 5 = 124.
3
Reduce 124 modulo 11 to standard non-negative remainder form
3
Dividing 124 by 11 yields a quotient of 11 with a remainder of 3 (124=11×11+3124 = 11 \times 11 + 3).

Key Concept

Modular Exponentiation and Algebraic Evaluation in Modular Arithmetic
Question 7Question →

What is the value of (17−42)(mod9)(17 - 42) \pmod{9} expressed in standard non-negative remainder form?

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Answer: 2

Answer

The value of (17−42)(mod9)(17 - 42) \pmod{9} in standard non-negative remainder form is 22.
Evaluating 17−4217 - 42 gives −25-25. Reducing −25-25 modulo 99 requires finding a non-negative remainder rr such that −25=9q+r-25 = 9q + r with 0≤r<90 \le r < 9. Choosing q=−3q = -3 yields −25=9(−3)+2-25 = 9(-3) + 2, making the correct non-negative remainder 22. Alternatively, 17≡8(mod9)17 \equiv 8 \pmod{9} and 42≡6(mod9)42 \equiv 6 \pmod{9}, so 8−6=2(mod9)8 - 6 = 2 \pmod{9}.

Step-by-Step Solution

1
Perform the subtraction within the parentheses.
17−42=−2517 - 42 = -25
Evaluate the arithmetic expression inside the modulo operation first.
2
Express −25-25 in terms of the modulus 99 using the division algorithm a=qn+ra = qn + r, where 0≤r<n0 \le r < n.
−25=9×(−3)+2-25 = 9 \times (-3) + 2
The remainder rr must be non-negative (0≤r<90 \le r < 9).
3
Identify the canonical non-negative remainder.
r=2r = 2
Alternatively, −25≡−7(mod9)-25 \equiv -7 \pmod{9}, and adding the modulus gives −7+9=2-7 + 9 = 2.

Key Concept

Modular Arithmetic and Negative Remainder Reduction
Estimated Time:45s
Question 8Question →

A binary operation Δ\Delta defined on the set of integers modulo 1111 is given by aΔb≡(2a2−3ab+b2)(mod11)a \Delta b \equiv (2a^2 - 3ab + b^2) \pmod{11}. Find the smallest non-negative integer xx that satisfies the equation 4Δx≡5(mod11)4 \Delta x \equiv 5 \pmod{11}.

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Answer: 3

Answer

The smallest non-negative integer xx that satisfies 4Δx≡5(mod11)4 \Delta x \equiv 5 \pmod{11} is 3.
Evaluating 4Δx4 \Delta x gives 32−12x+x2≡x2−x+10(mod11)32 - 12x + x^2 \equiv x^2 - x + 10 \pmod{11}. Setting this congruent to 5(mod11)5 \pmod{11} yields x2−x+5≡0(mod11)x^2 - x + 5 \equiv 0 \pmod{11}, which converts to x2−x−6≡0(mod11)x^2 - x - 6 \equiv 0 \pmod{11}. Factoring gives (x−3)(x+2)≡0(mod11)(x - 3)(x + 2) \equiv 0 \pmod{11}, which yields solutions x≡3(mod11)x \equiv 3 \pmod{11} and x≡9(mod11)x \equiv 9 \pmod{11}. The smallest non-negative integer among these solutions is 33.

Step-by-Step Solution

1
Substitute the given value a=4a = 4 into the operation definition.
4Δx=2(4)2−3(4)x+x2=32−12x+x24 \Delta x = 2(4)^2 - 3(4)x + x^2 = 32 - 12x + x^2
This establishes the explicit algebraic polynomial in terms of xx.
2
Reduce coefficients modulo 1111.
32≡10(mod11)32 \equiv 10 \pmod{11} and −12x≡−x(mod11)-12x \equiv -x \pmod{11}, giving x2−x+10(mod11)x^2 - x + 10 \pmod{11}.
Simplifying coefficients reduces computational complexity during equation solving.
3
Form the modular quadratic equation and set it to zero.
x2−x+10≡5(mod11)  ⟹  x2−x+5≡0(mod11)  ⟹  x2−x−6≡0(mod11)x^2 - x + 10 \equiv 5 \pmod{11} \implies x^2 - x + 5 \equiv 0 \pmod{11} \implies x^2 - x - 6 \equiv 0 \pmod{11}.
Expressing 5≡−6(mod11)5 \equiv -6 \pmod{11} allows standard integer factorisation.
4
Factor the quadratic polynomial and solve for xx.
(x−3)(x+2)≡0(mod11)  ⟹  x≡3 or x≡−2≡9(mod11)(x - 3)(x + 2) \equiv 0 \pmod{11} \implies x \equiv 3 \text{ or } x \equiv -2 \equiv 9 \pmod{11}.
Since 1111 is prime, a product congruent to 0(mod11)0 \pmod{11} implies at least one factor is congruent to 0(mod11)0 \pmod{11}.
5
Select the smallest non-negative integer from the valid solution set {3,9}\{3, 9\}.
x=3x = 3
33 is non-negative and strictly smaller than 99.

Key Concept

Modular Arithmetic Binary Operations and Quadratic Congruences
Estimated Time:3m 0s
Question 9Question →

What is the value of (−17×5)(mod6)(-17 \times 5) \pmod{6} expressed in standard non-negative remainder form?

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Answer: 55

Answer

55
Multiplying −17-17 by 55 yields −85-85. When −85-85 is divided by 66, the closest multiple of 66 below −85-85 is −90-90. The non-negative remainder is −85−(−90)=5-85 - (-90) = 5, which belongs to the canonical set {0,1,2,3,4,5}\{0, 1, 2, 3, 4, 5\}.

Step-by-Step Solution

1
Multiply the numbers inside the parentheses
−17×5=−85-17 \times 5 = -85
Perform standard multiplication before applying the modulo operation.
2
Express the negative number in terms of the modulus 6
−85=6×(−15)+5-85 = 6 \times (-15) + 5
Find the largest multiple of 6 less than or equal to -85, which is -90.
3
Extract the canonical non-negative remainder
55
In standard modular arithmetic, remainders must lie in the range [0,n−1][0, n-1], where nn is the modulus.

Key Concept

Modular Arithmetic and Non-Negative Remainders
Question 10Question →

Find the value of rr such that −29≡r(mod8)-29 \equiv r \pmod{8}, where 0≤r<80 \le r < 8.

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Answer: 3

Answer

The value of rr in standard non-negative remainder form is 33.
Dividing −29-29 by 88 yields a quotient of −4-4 and a remainder of 33, because 8×(−4)+3=−298 \times (-4) + 3 = -29. Since 0≤3<80 \le 3 < 8, 33 is the standard non-negative remainder.

Step-by-Step Solution

1
Express −29-29 in the form 8q+r8q + r, where qq is an integer and 0≤r<80 \le r < 8.
−29=8(−4)+3-29 = 8(-4) + 3
To find the standard remainder modulo 88, the remainder rr must satisfy 0≤r<80 \le r < 8.
2
Identify the remainder value rr.
r=3r = 3
Since 33 lies within the required range [0,7][0, 7], it is the canonical remainder.

Key Concept

Modular Arithmetic and Non-Negative Remainders
Question 11Question →

Determine the least non-negative integer congruent to 68(mod9)68 \pmod{9}.

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Answer: 5

Answer

The least non-negative integer congruent to 68(mod9)68 \pmod{9} is 5.
Dividing 68 by 9 gives a quotient of 7 and a remainder of 5. Since 0≤5<90 \le 5 < 9, the value 5 is the standard non-negative remainder.

Step-by-Step Solution

1
Divide 68 by the modulus 9 using the division algorithm.
68=9×7+568 = 9 \times 7 + 5
Every integer aa can be written uniquely as a=nq+ra = nq + r where qq is the quotient and 0≤r<n0 \le r < n.
2
Extract the non-negative remainder rr.
r=5r = 5
In modular arithmetic, the least non-negative integer congruent to a(modn)a \pmod{n} is the remainder rr when aa is divided by nn.

Key Concept

Modular Arithmetic Remainder
Question 12Question →

What is the smallest non-negative integer xx that satisfies the modular congruence 5x+7≡2(mod9)5x + 7 \equiv 2 \pmod{9}?

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Answer: 88

Answer

The smallest non-negative integer xx is 8.
Subtracting 77 from both sides yields 5x≡−5(mod9)5x \equiv -5 \pmod{9}, which reduces to 5x≡4(mod9)5x \equiv 4 \pmod{9}. Multiplying both sides by the modular multiplicative inverse of 55 modulo 99 (which is 22, since 5×2=10≡1(mod9)5 \times 2 = 10 \equiv 1 \pmod{9}) gives x≡4×2=8(mod9)x \equiv 4 \times 2 = 8 \pmod{9}. Thus, the smallest non-negative integer value is 88.

Step-by-Step Solution

1
Isolate the term containing xx by subtracting 7 from both sides of the congruence.
5x≡2−7≡−5(mod9)5x \equiv 2 - 7 \equiv -5 \pmod{9}
Standard algebraic balance operations apply under modular arithmetic.
2
Convert the negative remainder −5-5 to its canonical non-negative representative in modulo 9.
−5+9=4  ⟹  5x≡4(mod9)-5 + 9 = 4 \implies 5x \equiv 4 \pmod{9}
Modular numbers are conventionally expressed in the non-negative range [0,n−1][0, n-1].
3
Find the modular multiplicative inverse of 5 modulo 9 and multiply both sides by it.
Since 5×2=10≡1(mod9)5 \times 2 = 10 \equiv 1 \pmod{9}, the inverse is 2. Multiplying gives x≡4×2=8(mod9)x \equiv 4 \times 2 = 8 \pmod{9}.
Multiplying by the modular inverse solves for xx when gcd⁡(5,9)=1\gcd(5, 9) = 1.

Key Concept

Linear Modular Congruences and Modular Inverses
Estimated Time:1m 30s
Question 13Question →

If xx is the least positive integer satisfying the modular congruence 4x+9≡3(mod11)4x + 9 \equiv 3 \pmod{11}, what is the value of (x2−5x+2)(mod11)(x^2 - 5x + 2) \pmod{11} expressed in standard non-negative remainder form?

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Answer: 9

Answer

The value of (x2−5x+2)(mod11)(x^2 - 5x + 2) \pmod{11} in standard non-negative remainder form is 9.
Solving 4x+9≡3(mod11)4x + 9 \equiv 3 \pmod{11} yields 4x≡−6≡5(mod11)4x \equiv -6 \equiv 5 \pmod{11}. Multiplying by 3 (the inverse of 4 mod 11) gives x≡15≡4(mod11)x \equiv 15 \equiv 4 \pmod{11}. Evaluating (x2−5x+2)(x^2 - 5x + 2) at x=4x = 4 gives 16−20+2=−216 - 20 + 2 = -2. Converting −2-2 into the standard non-negative remainder range [0,10][0, 10] gives −2+11=9-2 + 11 = 9.

Step-by-Step Solution

1
Isolate the variable term in the linear modular congruence
4x≡3−9≡−6(mod11)4x \equiv 3 - 9 \equiv -6 \pmod{11}
Subtract 9 from both sides of the congruence.
2
Convert the negative right-hand side to a non-negative residue modulo 11
4x≡−6+11≡5(mod11)4x \equiv -6 + 11 \equiv 5 \pmod{11}
Add the modulus 11 to obtain the canonical non-negative equivalent.
3
Solve for xx by multiplying by the multiplicative inverse of 4 modulo 11
Since 4×3=12≡1(mod11)4 \times 3 = 12 \equiv 1 \pmod{11}, multiply both sides by 3: x≡5×3=15≡4(mod11)x \equiv 5 \times 3 = 15 \equiv 4 \pmod{11}. Thus, the least positive integer is x=4x = 4.
The modular inverse of 4 modulo 11 is 3.
4
Substitute x=4x = 4 into the expression (x2−5x+2)(x^2 - 5x + 2)
42−5(4)+2=16−20+2=−24^2 - 5(4) + 2 = 16 - 20 + 2 = -2
Evaluate the quadratic expression using the calculated value of xx.
5
Express −2-2 in canonical non-negative remainder form modulo 11
−2+11=9(mod11)-2 + 11 = 9 \pmod{11}
Add the modulus 11 to convert the negative result to a non-negative remainder within [0,10][0, 10].

Key Concept

Solving linear modular congruences and converting negative remainders to canonical form
Question 14Question →

What is the canonical non-negative remainder when 345−113^{45} - 11 is divided by 77?

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Answer: 2

Answer

The correct answer is 2.
Using modular exponentiation, 33=27≡−1(mod7)3^3 = 27 \equiv -1 \pmod{7}, so 345=(33)15≡(−1)15=−1(mod7)3^{45} = (3^3)^{15} \equiv (-1)^{15} = -1 \pmod{7}. Subtracting 11 yields −1−11=−12(mod7)-1 - 11 = -12 \pmod{7}. Adding 14 (a multiple of 7) gives −12+14=2-12 + 14 = 2, which is the canonical non-negative remainder.

Step-by-Step Solution

1
Evaluate 345(mod7)3^{45} \pmod{7} using power rules
345≡−1(mod7)3^{45} \equiv -1 \pmod{7}
Since 33=27≡−1(mod7)3^3 = 27 \equiv -1 \pmod{7}, raising both sides to the power of 15 gives (33)15≡(−1)15=−1(mod7)(3^3)^{15} \equiv (-1)^{15} = -1 \pmod{7}.
2
Subtract 11 from the modular result
-12 \pmod{7}
Replacing 3453^{45} with −1-1 gives −1−11=−12-1 - 11 = -12.
3
Reduce -12 to its canonical non-negative remainder in [0,6][0, 6]
2
Adding the smallest multiple of 7 that makes the result non-negative gives −12+14=2-12 + 14 = 2.

Key Concept

Modular Exponentiation and Negative Remainder Reduction
Question 15Question →

What is the value of (−43×15)(mod8)(-43 \times 15) \pmod{8} expressed in standard non-negative remainder form?

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Answer: 33

Answer

3
Reducing −43-43 modulo 88 gives 55, and 1515 modulo 88 gives 77. Multiplying these results yields 35≡3(mod8)35 \equiv 3 \pmod{8}, which is the canonical non-negative remainder.

Step-by-Step Solution

1
Reduce individual factors modulo 8
−43≡5(mod8)-43 \equiv 5 \pmod{8} (since −43=−6×8+5-43 = -6 \times 8 + 5) and 15≡7(mod8)15 \equiv 7 \pmod{8} (since 15=1×8+715 = 1 \times 8 + 7)
Simplifying terms before multiplication makes arithmetic easier.
2
Multiply the reduced remainders
5×7=355 \times 7 = 35
Modular arithmetic preserves multiplication.
3
Reduce the product modulo 8 to obtain the canonical non-negative remainder
35=4×8+3  ⟹  35≡3(mod8)35 = 4 \times 8 + 3 \implies 35 \equiv 3 \pmod{8}
The final answer in modular arithmetic must fall within the range [0,n−1][0, n-1].

Key Concept

Modular Arithmetic and Canonical Non-Negative Remainders
Question 16Question →

If xx is the smallest non-negative integer satisfying the modular congruence 7x+4≡2(mod13)7x + 4 \equiv 2 \pmod{13}, find the value of (x3+2x)(mod13)(x^3 + 2x) \pmod{13} expressed as a canonical non-negative remainder.

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Answer: 6

Answer

The canonical non-negative remainder is 6.
Subtracting 4 from both sides of 7x+4≡2(mod13)7x + 4 \equiv 2 \pmod{13} gives 7x≡−2≡11(mod13)7x \equiv -2 \equiv 11 \pmod{13}. Multiplying by the modular inverse of 7 (which is 2, since 7×2=14≡1(mod13)7 \times 2 = 14 \equiv 1 \pmod{13}) yields x≡22≡9(mod13)x \equiv 22 \equiv 9 \pmod{13}. Evaluating (93+2×9)(mod13)(9^3 + 2 \times 9) \pmod{13} gives (729+18)=747(729 + 18) = 747. Dividing 747 by 13 gives a quotient of 57 and a remainder of 6.

Step-by-Step Solution

1
Isolate the linear term in the congruence
7x≡−2≡11(mod13)7x \equiv -2 \equiv 11 \pmod{13}
Subtracting 4 from both sides simplifies the equation, and −2+13=11-2 + 13 = 11 converts the negative remainder to positive form.
2
Solve for xx by multiplying by the multiplicative inverse of 7 modulo 13
x≡9(mod13)x \equiv 9 \pmod{13}, so x=9x = 9
Since 7×2=14≡1(mod13)7 \times 2 = 14 \equiv 1 \pmod{13}, multiplying 7x≡11(mod13)7x \equiv 11 \pmod{13} by 2 yields x≡22≡9(mod13)x \equiv 22 \equiv 9 \pmod{13}.
3
Evaluate (x3+2x)(mod13)(x^3 + 2x) \pmod{13} using modular reduction
66
93=729=56×13+1≡1(mod13)9^3 = 729 = 56 \times 13 + 1 \equiv 1 \pmod{13} and 2×9=18=1×13+5≡5(mod13)2 \times 9 = 18 = 1 \times 13 + 5 \equiv 5 \pmod{13}. Adding these gives 1+5=61 + 5 = 6.

Key Concept

Solving linear modular congruences and modular polynomial evaluation
Estimated Time:1m 30s
Modular Arithmetic Practice Questions — JAMB UTME | Examkin